We propose a tool to engineer cancer or host cells to irreversibly mark nearby cells by means of novel exosomal Cre recombinase constructs. By creating cancer cells that transport Cre, the proximal microenvironment in Cre-responsive animals can be analyzed or manipulated to an extent well beyond our current capabilities. These sorts of manipulations would enable the detailed dissection of molecular mechanisms that shape the directionality of cancer spread, sustain dormancy or control cancer-immune cell interactions. Exosomes are nanovesicles that are robustly produced by cancer cells. Specific tags directing Cre to cancer exosomes will generate a visual (fluorescent) map of the trajectory that the cancer cells used during metastasis in fluorescent reporter animals. This will also provide a means to isolate each cell that contacted the cancer so that the transcriptome of those cells can be compared with that of similar cells that were nave to the cancer. This strategy is expected to offer an unprecedented tool for the dissection of the lung cancer microenvironment that is engaged during the metastatic process. Specific Aims of this application are to: 1. Create and evaluate tools that package fluorescent Cre-recombinase into exosomes for delivery to adjacent cells. This aim will establish the optimal construct for Cre transfer between cancer and surrounding normal cells. We will construct fusion genes that combine Cre, a red fluorescent marker, and specific trafficking domains of exosomal proteins. Exosome-specific Cre activity and transfer will be confirmed. Fluorescent conversion of GFP-reporter cells along the path of migrating e-C cancer cells will be visualized through live cell imaging and quantitatively evaluated. Aim 2. Enable and evaluate niche-specific activity of Cre-transfer by hypoxic cancer cells in vivo. This aim will demonstrate microenvironment-specific transfer of Cre from cancer to bystander cells. Luciferase-expressing Lewis lung carcinoma cells (LL2/luc- M38) will be stably transfected with red fluorescent exosomal Cre under control of the VEGF promoter 6, 7. Metastases following intravenous inoculation of syngeneic Cre-reporter mice will be visualized using whole animal imaging. Ex vivo multiphoton confocal microscopy will then measure host cell conversion to green fluorescence at candidate hypoxic regions and along metastatic tracks. Pimonidazole injection prior to sacrifice will be used to validate hypoxia in candidate regions. The ability to capture and analyze Cre-targeted peritumoral cells will be tested. Impact: This tool will allow robust collection of imaging and cellular data that unambiguously delineate cancer/bystander cell interactions that occurred in vivo in desired microenvironments. It is expected to create an unprecedented historical record of each cell that touched a cancer cell. This record could illuminate functional changes that occur in the microenvironment that impact metastases, and how these changes are associated with alteration in the route of spread of cancer.